Last week, with a fair degree of razzmatazz and press coverage, Microsoft launched a smart thermostat called Glas. Except it wasn’t really Microsoft’s. And whilst it might be pretty, it certainly isn’t smart.

If you look behind the promotional video, it’s clear that it’s not really driven by any desire to be smart. It’s come out of Johnson Controls, who have been designing dumb thermostats for many years, and it perpetuates the dumb elements of control, which means it won’t save users as much money as a proper smart device could. However, small things like the truth didn’t stop them headlining it as “reinventing the thermostat”. I suspect the only reason that Glas exists is that Microsoft are currently in a poor third place in getting their Cortana speech recognition capability into the market. I quite like Cortana, but compared with Amazon and Google’s success in persuading consumer product manufacturers to support their offerings, Cortana is definitely an also-ran.

What you see if you watch the video carefully is an outdated control system, a user interface that was probably inspired by Bishop Berkeley and an attempt to break the second law of thermodynamics. All of which details appear to have slipped past the rose-tinted editorial glasses of the technology press, who have just said “Shiny – want one!”. So let me explain why it’s another smart opportunity missed.

Over the years, I’ve been involved in designing a number of heating control systems. In my first post-college job, it was a vacuum annealing furnace at Marconi; fifteen years later it was a range of sperm and embryo freezers for Planer Products and more recently, I was involved with a smart thermostat, while I was CTO at Onzo. The smart thermostat was a project for SSE – the UK’s second largest energy supplier. It coupled in to Onzo’s cloud-based appliance disaggregation algorithms, providing a level of intelligent control that I’ve still to see matched in any other product. However, SSE decided that nobody would be interested in using a smart thermostat and cancelled the development. Three months later, in October 2011, Nest launched their smart thermostat. Which tells you a lot about the intelligence and market awareness of energy suppliers.

One thing we discovered during that project is that is that you shouldn’t display temperature. Every thermostat does, yet if you ask a group of people what the temperature of a room is, they’ll all have a different number. The fact is that we’re far more interested in comfort, and that doesn’t relate to a specific temperature, as our perception of temperature varies throughout the day. It’s not just related to activity; you tend to feel warmer when your blood pressure is highest in the late afternoon, and then cooler as the evening draws on, as your circadian cycle progresses. That means that if you want to feel the same level of warmth throughout the day, the room temperature needs to move several degrees to match your perception. But thermostat designers ignore this, because they’ve been taught to concentrate on numbers, despite the fact it means very little to anyone. It would be smart if Microsoft had recognised that, but if you look at the headline photo for the Glas, it’s all about the number:

When you look at it more closely, you notice something odd. The temperature is currently 70°F, but the heating is set to 68°F and the cooling is set to 72°F. In other words, it implies that it wants the aircon temperature to be higher than the heating temperature. Anyone remembering their school physics should remember that the second law of thermodynamics states that you can’t take heat from a colder to a hotter state, but this is what the Glas appears to be doing. (If you don’t remember the second law of thermodynamics, I can recommend the comic song about it from Flanders and Swann).

Has Microsoft found a way to break the laws of thermodynamics? No – it’s actually an indication that they’re using one of the most inefficient forms of heating control that exists. To see why, we need to have a quick look at control theory. But I’ll try to make it easy.

If you have just got a heating system and want to warm the room up, you set the temperature you want – let’s say 20°C, and once you turn the heating on, you expect the temperature to gradually increase to that temperature, as shown below.

For that to happen, the heater needs to start turning off before it gets to 20°C, otherwise it will overshoot. In practice, domestic heating controls aren’t that clever. Most heating systems can only be ON or OFF, so they shoot up to 20°C at full power, then turn off. Once the temperature falls back down to 20°C they’ll turn on again, so you end up with the actual room temperature oscillating around what you want. How much the temperature oscillates depends on the outside temperature and how well your house is insulated.

There is no reason why you can’t control the heat input to prevent this. All you need to do is to cut back the amount of heating by turning off the boiler (furnace if you’re in the US) before you reach 20°C, then turning it on occasionally to maintain the temperature. It’s called Proportional Control and is the basis of every decent heating or cooling system, but not very many domestic thermostats.

It’s not used because it is more complex and would add a few cents to the cost of the thermostat. Over the life of the thermostat it would save you hundreds or thousands of dollars in energy savings, but thermostat manufacturers are only interested in those few cents. They get away with it, because most of the time you don’t notice the oscillating temperature. They like the ON / OFF scheme, as it’s really simple, which is why basic thermostat design hasn’t really changed for the past century. Where it gets more complicated, and starts wasting a lot of energy is when you add in air conditioning.

If you look at the Glas display again, it’s not really breaking the second law of thermodynamics. Instead it’s told the heating to turn off at 68°F, which is where it will start to overshoot and get too warm, but then tells the aircon to cut in at 72°F to limit the overshoot and cool the room down again.

The upshot of this is that instead of just running your heating if it’s cold, or just running the aircon if it’s hot, you end up using both to compensate for each other’s poor control, using far more energy than you need. In general, you should only ever combine heating and cooling if you want to control the rate of temperature change. That’s important in annealing furnaces and sperm freezers, but it’s not relevant for home heating.

The diagram below shows the effect of these different approaches on total energy usage. Proportional control starts backing off the heating as the room comes up to temperature and then turns it on for small periods to keep the house at a constant temperature. That means it’s very energy efficient. Simple ON/OFF heating control always heats up too much, so energy is wasted in overshooting. But mixing heating and cooling makes it far worse, as both heating and cooling get turned on to compensate for each of their overshooting. The more white space you see in the diagram, the more efficient your energy usage is.

A smart thermostat can easily learn about the thermal response of the house, i.e. work out how quickly it warms up or cools down, and use this knowledge, along with weather forecasts, to implement proportional control. But it’s obvious from the display that Glas doesn’t do that. It means that their claim about saving you energy is rather hollow. It may help save some if it’s easier to set up than a traditional thermostat. Most thermostats have such an impenetrable user interface that the user never changes the programme to match their personal requirements, which wastes energy. However, there’s nothing difficult or smart about solving that problem – it’s just a case of employing a decent user interface designer. However, it doesn’t look as if that was a priority during the Glas development. To see why, let me show you another screen.

The Glas includes a sensor which detects whether you’re in the house. Unless you have a very small house it will probably get that wrong, as it’s only detecting the room it’s placed in, but some brain-dead User Interface designer has considered it is a good idea to have a display to tell you you’re not there to watch it. Just look at the top right segment, which is usefully informing you that you’re not there. This is the taking UI design to the Bishop Berkeley level. What is the point of telling me I’m not there? You also have to assume that the occupancy sensor doesn’t work, otherwise how would you be able to take this photograph? Unless you’re a burglar standing outside the window with a telephoto lens? If there were a stupid Olympics, the Glas would be up there on the podium taking the stupid prize.

It shouldn’t be difficult to design a thermostat which is easy to use and which helps save energy, but the industry does seem to find it a near impossible challenge. It’s why the vast majority of users never adjust their heating controls from the day they’re installed. It’s also why so much energy is wasted, which makes it a scandal that companies like Johnson Controls, Honeywell, Emerson, Drayton and others don’t do a better job. Glas certainly fails, and from the video it looks as if it may claim the crown of being the world’s most stupid thermostat. It’s probably not entirely Microsoft’s fault – I suspect the main specification came from Johnson Controls, but by promoting themselves in the video, Microsoft are doing a pretty good job of showing they don’t have much of a clue. It’s another reminder that even the giants of the industry still have very little understanding of what smart home or consumer IoT means. In this case, Microsoft really should have known better. Instead of a smart thermostat, they have merely produced something styled for consumers and journalists who are even less smart than their homes. And doing it just because you’re scared that nobody wants to talk to your voice assistant is really not a credible excuse.

As long as there’s decent airflow over the sensor, which is representative of the state of the room, and not affected by other heat sources, such as direct sunlight, it doesn’t matter too much. The important thing is that it needs to be repeatable. That’s why the display of temperature really isn’t very important. There could be several degrees between where you normally sit and the sensor location, but as long as it controls where you sit to the same temperature it doesn’t matter. Once you display a temperature, people start saying “it’s too high”, or “it’s too low”, rather than “does it feel right”.

The same applies to the thermostatic valve. As long as they’re consistent, you’re fine.

I have seen a couple of smart thermostat designs where the heat generated by the electronics within them distorts the temperature reading, It’s probably why so many smart thermostats display the setpoint rather than the actual temperature.

Nice critique.
But it’s even worse: american thermostats do not support the OpenTherm (modulating) protocol.
Most central heating systems in middle and northern Europe can be controlled much better than just on/off.
When your house approaches the set temperature, the thermostat can tell the system to heat at 10-90% of full power.
Especially maintaining a constant temperature can be much more precise and economical in this way.
Few ‘smart’ thermostats support this.
The Nest was famously lacking this until the most recent version.
The thermostat I like best and have in my home is the Toon. It also lets connect your smart energy meter or sensors that monitor your electricity / gas meter.
Have a look at https://www.toon.eu

By not having any smart, IoT heating control system I can avoid the following:
• Spending £££££
• The environmental cost of semiconductor products and their throw-away lifespan
• Having to understand the UI (which could change after a software update)
• Having the system hacked
• Requiring software updates
• Requiring a reboot to function
• Becoming obsolete after a couple of years
• Being connected to an external company that could go bust or decide to withdraw the service at any time

I just don’t understand the fuss..…

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About Creative Connectivity

Creative Connectivity is Nick Hunn's blog on aspects and applications of wireless connectivity. Having worked with wireless for over twenty years I've seen the best and worst of it and despair at how little of its potential is exploited.

I hope that's about to change, as the demands of healthcare, energy and transport apply pressure to use wireless more intelligently for consumer health devices, smart metering and telematics. These are my views on the subject - please let me know yours.

Essentials of Short Range Wireless

A helping hand for wireless designers

Adding wireless connectivity to a product is a major challenge for any designer. There are so many new concepts, and a plethora of suppliers claiming they’ve solved them for you. I’ve tried to distil 20 years of experience into this book to help you get over the pitfalls, ask the right questions and make sure you understand the answers.